Resistivity Imaging Using Phase Sensitive Detection with a Floating Reference Signal

ABSTRACT

A device, method and system for measuring characteristics of a geologic formation using a floating reference signal having a mud chamber, an electrode disposed within the mud chamber, and an electrically conductive plate disposed within the mud chamber, the plate separated from the electrode. An alternating current source is provided on the electrode, whereby an electric field is be maintained between the electrode and the conductive plate. An opening in the mud chamber allows drilling fluids to pass there through.

STATEMENT OF RELATED CASES

This application claims the benefit of U.S. Provisional Application No.60/966,707, filed Aug. 29, 2007.

FIELD OF THE INVENTION

The present invention relates generally to a device, method, and systemfor the downhole measurement of the characteristics of a geologicformation and, in a particular though non-limiting embodiment, to adevice, method, and system for resistivity imaging in oil-base mud.

BACKGROUND OF THE INVENTION

Galvanic imaging devices utilizing impedance measurements in oil-basemud are very sensitive to the oil-filled gap between the electrodesurface and the geologic formation. Multiple attempts have been made toeliminate the problem and are known in the prior art.

Typically, an imaging tool contains a series of small buttons mounted ona metal pad and separated by narrow insulating gaps. The buttons serveas electrically conducting electrodes. In oil-base mud, the measuredimpedance of individual buttons depends to a large extent on the mudcake parameters. In addition, an oil film on the pad surface maycompletely eliminate the electrical contact between pad and formation.

In conductive mud, the mud cake has low resistivity and, consequently,is almost transparent to the current flowing through it. In oil-basemud, the mud cake is very resistive, contributing greatly to themeasured ground resistance. Therefore, the true value of formationresistivity is significantly obscured. In addition, a thin oil film maycover the surface of the pad, making the overall ground resistance solarge that it is practically impossible to inject sufficient currentinto the formation.

The size of a button is associated with the tool spatial resolution.Usually, the button radius is in the range of 1 to 2 mm, creating a verylarge ground resistance. For example, a 2 mm button on a typicalfocusing pad has a ground resistance of 10,000 Ohm in a 1 Ohm-mformation or 10,000,000 Ohm in a 1,000 Ohm-m formation. This illustratesthe technical challenge of producing a high definition image in aresistive-mud environment.

In the present invention, a new principle is introduced based on phasesensitive detection with the phase established with respect to afloating reference. The floating reference represents the electric fieldin the gap. Mud-filled chambers in front of the electrodes are used tomeasure the reference field.

SUMMARY OF THE INVENTION

According to a first set of examples of the present invention, there isprovided a device for measuring characteristics of a geologic formationusing a floating reference signal, the device including: a mud chamber;an electrode disposed within the mud chamber; an electrically conductiveplate disposed within the mud chamber, the plate separated from theelectrode, whereby an electric field may be maintained between theconductive plate and the electrode; and an opening in the mud chamber,thereby allowing drilling fluids to pass there through.

According to another example of the invention, the above-describeddevice further comprises an alternating current source, the alternatingcurrent source in electrical connection with the electrode.

According to another example of the invention, the plate of theabove-described device is an electrically conductive lattice.

According to another example of the invention, the distance ofseparation between the electrode and the plate is less than the smallestlineal surface dimension of the electrode.

According to another example of the invention, there is provided adevice for measuring characteristics of a geologic formation in awellbore using a floating reference signal, the device including: aplurality of electrodes; a plurality of electrically conductive plates,each of the plurality of electrically conductive plates positionedrespective to at least one of the plurality of electrodes, where theplates of the plurality of electrically conductive plates are separatedfrom each other by an insulating material; and a mud chamber locatedbetween the plurality of electrodes and the plurality of electricallyconductive plates. The plurality of electrodes and the plurality ofplates are electrically isolated from each other except through anycontents contained in the mud chamber and the mud chamber is in fluidcommunication with the wellbore, allowing wellbore fluids to pass therethrough.

According to a second set of examples of the present invention, there isprovided a method for characterizing a geologic formation including thesteps of: determining the electrical properties of a current introducedinto the geologic formation from a wellbore, thereby producing aformation measurement; determining, through a portion of the wellbore,the electrical properties of that current introduced into the geologicformation, thereby producing a reference signal; and differentiating theproduced formation measurement from the produced reference signal.

According to another example of the invention, the differentiation stepof the above-described method includes the step of determining the phasedifference between the produced formation measurement and the producedreference signal.

According to another example of the invention, the step of determiningthe phase difference between the formation measurement and the referencesignal includes the step of measuring the voltage of the introducedcurrent when the voltage of the reference signal is approximately zero.

According to another example of the invention, the differentiation stepof the above-described method comprises: determining the impedanceamplitude of the produced formation measurement.

According to another example of the invention, the differentiation stepof the above-described method includes: determining the phase differencebetween the formation measurement and the reference signal, whereby ameasurement of the phase difference is produced; and selecting betweenmeasurement of the phase difference between the formation measurementand the reference signal for large phase differences and measurement ofthe impedance amplitude of said formation measurement for small phasedifferences, producing a characterization of the geologic formation.

According to another example of the invention, the introduced current isprovided through at least one of a plurality of electrodes disposed inthe wellbore. In another example, the introduced current is analternating current.

According to another example of the invention, the step of determiningthe electrical properties of a current introduced into the geologicformation from a wellbore includes the steps of: introducing analternating electrical current into the wellbore using an electrode inelectrical contact with the wellbore fluids, thereby inducing a currentinto the geologic formation; measuring the alternating voltage of theintroduced alternating electrical current, thereby producing a firstmeasured voltage; and measuring the current of the introducedalternating electrical current, thereby producing a first measuredcurrent. The step of determining the electrical properties of thecurrent introduced into the geologic formation through a portion of saidwellbore includes the step of measuring the differential alternatingvoltage of the introduced alternating electrical current between theelectrode and an electrically conductive plate disposed between theelectrode and the geologic formation, thereby producing a secondmeasured voltage.

In a further example, the step of differentiating the formationmeasurement from the reference signal includes the step of correlatingthe first measured voltage with the second measured voltage, therebyproducing a phase shift measurement. In another example, the step ofdifferentiating the formation measurement from the reference signalincludes the step of dividing the first measured current into the firstmeasured voltage when the non direct current component of the secondmeasured voltage is approximately zero. In another example, the step ofdifferentiating the formation measurement from the reference signalincludes the step of selecting at least between: correlating the firstmeasured voltage with the second measured voltage, thereby producing aphase shift measurement; and dividing the first measured current intothe first measured voltage when the non direct current component of thesecond measured voltage is approximately zero.

According to a third set of examples of the present invention, there isprovided a system for characterizing a geologic formation including: afirst determining means for determining the electrical properties of acurrent introduced into the geologic formation from a wellbore, therebyproducing a formation measurement; a second determining means fordetermining the electrical properties of that current introduced intothe geologic formation through a portion of the wellbore, therebyproducing a reference signal; and a means for differentiating theproduced formation measurement from the produced reference signal,thereby producing a characterization of the geologic formation.

According to another example of the invention, the differentiation meansof the above-described system comprises a means for measuring the phasedifference between the produced formation measurement and the producedreference signal.

According to another example of the invention, the differentiation meansof the above-described system includes a means for measuring theimpedance amplitude of the produced formation measurement. In a furtherexample of the invention, the means for measuring the impedanceamplitude of the produced formation measurement includes measuring thevoltage of that introduced current when the voltage of the producedreference signal is approximately zero.

According to another example of the invention, the differentiation meansof the above-described system includes a means for selecting between: ameans for measuring the phase difference between the produced formationmeasurement and the produced reference signal for large phasedifferences; and a means for measuring the impedance amplitude of theproduced formation measurement for small phase differences.

According to another example of the invention, the second determiningmeans of the above-described system includes: a mud chamber locatedbetween an electrode and a plate, the electrode and the plateelectrically isolated from each other except through any contentscontained in the mud chamber; and the mud chamber in fluid communicationwith the wellbore, thereby allowing wellbore fluids to pass therethrough. In a further example of the invention, the distance ofseparation between the electrode and the plate is less than the smallestlineal surface dimension of the electrode.

According to another example of the invention, the second determiningmeans of the above-described system includes: a mud chamber locatedbetween a plurality of electrodes and a plate, the plurality ofelectrodes and the plate electrically isolated from each other exceptthrough any contents contained in the mud chamber; and the mud chamberin fluid communication with the wellbore, thereby allowing wellborefluids to pass there through.

According to another example of the invention, the second determiningmeans of the above-described system includes: a mud chamber locatedbetween a plurality of electrodes and a plurality of electricallyconductive plates, the plurality of plates separated from each other byan insulating material; each of the plurality of plates positionedrespective to at least one of the plurality of electrodes; the pluralityof electrodes and the plurality of plates electrically isolated fromeach other except through any contents contained in the mud chamber; andthe mud chamber in fluid communication with the wellbore, therebyallowing wellbore fluids to pass there through. In a further example ofthe invention, the introduced current is provided through at least oneof the plurality of electrodes disposed in the wellbore. In a furtherexample of the invention, the introduced current is an alternatingcurrent.

According to another example of the invention, the first determiningmeans of the above-described system includes: introducing an alternatingelectrical current into the wellbore using an electrode in electricalcontact with the wellbore fluids, thereby inducing a current into thegeologic formation; measuring the alternating voltage of the introducedalternating electrical current, thereby producing a first measuredvoltage; and measuring the current of the introduced alternatingelectrical current, thereby producing a first measured current. Thesecond determining means includes: an electrically conductive platedisposed between the electrode and the geologic formation; and measuringthe differential alternating voltage of the introduced alternatingelectrical current between the electrode and the electrically conductiveplate, thereby producing a second measured voltage. In a further exampleof the invention, the differentiating means includes correlating thefirst measured voltage with the second measured voltage, therebyproducing a phase shift measurement. In a further example of theinvention, the differentiating means includes dividing the firstmeasured current into the first measured voltage when the non directcurrent component of the second measured voltage is approximately zero.In a further example of the invention, the differentiating meansincludes selecting at least between: correlating the first measuredvoltage with the second measured voltage, thereby producing a phaseshift measurement; and dividing the first measured current into thefirst measured voltage when the non direct current component of thesecond measured voltage is approximately zero.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

FIG. 1 is a schematic view of a representative imaging tool in awellbore having an oil-filled gap 7.

FIG. 2 is a schematic view of measurement of voltage potential U in anoil-filed gap 7 in a wellbore.

FIG. 3 is an electrical measurement graph illustrating phase differencesbetween introduced voltage V1 and voltage potential U in an oil-filedgap 7.

FIG. 4 is a side schematic view of an example of the present inventionin a wellbore 13 with geologic formation 6.

FIG. 5 is a side schematic view of an example of the present inventionillustrating a measurement of voltage potential U.

FIG. 6 is a side schematic view of an example of the present inventionillustrating a plurality of electrode 9 and plate pairs 10.

FIG. 7 is a front view of an example of a lattice 10′ used for plate 10.

FIG. 8 is a side schematic view of an example of the present inventionillustrating a plurality of electrode 9 and a single plate 10.

DETAILED DESCRIPTION

Each of FIGS. 1-8 illustrates a geologic formation characterizationsystem embodying various aspects of the present invention, though theseparticular embodiments are illustrated and described herein only forexemplary purposes. Moreover, variations of the geologic formationcharacterization system and methods of utilizing the same will becomeapparent to those of ordinary skill in the relevant structural andmechanical arts upon reading the following disclosure. Thus, the presentinvention is not to be considered limited to only the structures,systems, and methods described herein.

As illustrated in FIG. 1, an imaging tool 1 having small buttons 2 ismounted on a metal pad 3 and separated by narrow insulating gaps 4. Pad3 is pushed against the borehole wall 5. Borehole wall 5 is formed bypiercing geologic formation 6. An oil-filled gap 7 may remain betweenpad 3 and formation 6. A Voltage V1 is created on the face of pad 3,allowing a current I1 to be injected into formation 6. A current F,flowing from the periphery of pad 3 focuses currents I1 emanating frombuttons 2. The ground impedance Z of buttons 2 may be related to theformation resistivity, R_(t), as follows:

R _(t) =Z/K;Z=V/I  (Eq.1)

Here, K represents the button resistance in a uniform formation with 1Ohm-m resistivity, the K-factor. In oil-base mud, the measured impedanceof individual buttons strongly depends on the quality of pad contactwith the formation and the size of any oil-filled gap 7 which may exist.Eq. 1 may result in a resistivity significantly different from the trueformation resistivity.

Certain measurement principles may be established for detection offormation resistivity independent of the oil-filled gap thickness,resistivity, and other properties affecting the gap impedance. Forexample, it is assumed that the return electrode is very large comparedto the injection buttons 2 such that the ground impedance of the returnmay be neglected.

Additional principles are summarized in FIG. 2 and FIG. 3. An injectedvoltage V1 is introduced into button 2, injected voltage V1 oscillatingin time with a high frequency of ω=2πf and otherwise providing a currentI. All physical quantities follow a sinusoidal pattern in time withdifferent phase delays. The phase delays are functions of formationresistivity, tool design, and other characteristics of the overallsystem.

Any quantity, including voltage and current, can be measured at aparticular moment in time. The selection of time may be critical forproviding sensitivity of the measurement to the desired formationproperties or elements. For example, in induction logging, themeasurements of the magnetic field are performed at such a moment whenthe current in the transmitter equals to zero. Such a selection of timefor measurements removes a very strong “primary field” from the signaland provides the best sensitivity of the measured field to formationconductivity. The transmitter current is said to be “a referencesignal”, and the measurements are considered to be performed“out-of-phase” with the reference signal. An important property of thereference signal in induction logging is that the reference signal doesnot depend on the formation properties, exhibiting a very high accuracyover a range of variation in formation properties.

In the present invention, it is optimum to perform measurements of thecurrent and voltage at such a time when the voltage U across theoil-filled gap 7 equals zero. At such a moment, the impedance of buttons2 does not depend on the properties of gap 7, including the thickness,resistivity, and dielectric permittivity of gap 7. At such moment,voltage U may be used as a reference.

Selecting voltage U as a reference eliminates the influence ofoil-filled gap 7 directly in the measurements. However, voltage U is noteasy to measure. In principle, what needs to be measured and used as areference is the electric field in gap 7. Conceivably, to achieve such ameasurement, a small electric antenna could be disposed perpendicular tothe electrode surface. If such an antenna, or any other device capableof detecting electric field in gap 7, is feasible or exists then theproposed measurements will be almost free of the influence of gap 7. Itis worth noting that the antenna should not necessarily touch formation6.

Unlike the reference current in induction logging, a reference signalfor galvanic imaging depends on the properties of formation 6 and on theproperties of oil-filled gap 7. Reference signal variations are able toautomatically compensate for the gap variations, providing the impedancemeasurements can be made that are much more sensitive to the formationresistivity.

A feasible way of measuring the reference signal is illustrated in FIG.4. Mud chamber 8 is placed in front of measurement electrodes 9. CurrentI is introduced on electrode 9. Chamber 8 is confined between thesurface of electrode 9 and a metal plate 10 placed in front of electrode9. Plate 10 and electrode 9 are mounted on an insulating support frame11. Plate 10 and electrode 9 are electrically isolated from each other.Mud chamber 8 has open channels 12 allowing for the access of mud 14from the borehole 13. Mud 14 between electrode 9 and metal plate 10 atevery logging depth is the same as in the surrounding mud in borehole13. Other mechanisms for providing mudflow through chamber 8 may beenvisioned. Preferably, the radial extent of chamber 8 is smaller thanthe axial and azimuthal size of electrodes 9. Under such conditions, theelectric field at the location of plate 10 will be almost normal to thesurface of plate 10, and consequently, metal plate 10 will betransparent with respect to the field produced by electrode 9. Theelectric field produced by electrode 9 in the presence of plate 10 isalmost the same as without the plate.

As shown in FIG. 5, plate 10, together with electrode 9, provides areference for measurement of the desired electric field U_(ref) inchamber 8, representing the oil-filled gap.

As shown in FIG. 6, in a further example, individual references for eachelectrode 9 may be provided in a multi-electrode arrangement. CurrentsI1, I2, I3, I4 are introduced on each respective electrode 9. Metalplates 10 are embedded in insulating frame 11 in front of respectiveelectrodes 9. All chambers 8 are connected via mud channels 12 such thatmud 14 is able to flow through all chambers. Electric field referencesU1 _(ref), U2 _(ref), U3 _(ref), U4 _(ref) may be measured on eachrespective pair of electrodes 9 and plates 10.

At high frequencies, depending on the thickness of metal plate 10, theskin-effect may be a potential problem. In a further example, a metallattice 10′, as illustrated in FIG. 7, is be used for plate 10 insteadof a solid plate. In another example, such a lattice may is also used toimprove the mud circulation through chambers 8.

As illustrated in FIG. 7 as a further example, a single common plate 10″for all electrodes 9 is used.

Mud chamber 8 reduces the overall capacitance in front of each electrode9 because two capacitors are connected in series, that of mud chamber 8and that of gap 7 between metal plate 10 and formation 6. The reducedcapacitance increases overall impedance of each electrode 9. However,making the size of mud chamber 8 small in the radial direction mayeffectively control this effect.

The phase shift, Δφ, between the current injected through electrode 9and the electric field in gap 7 exactly equals, with an opposite sign,the phase of complex conductivity of the mud:

tg(Δφ)=ω∈/σ  (Eq. 2)

Here, ω equals 2πf, which represents the frequency of the introducedcurrent and ∈ represents the dielectric permittivity of the mud and σrepresents the electrical conductivity of the mud. Eq. 2 follows fromthe assumption that the displacement currents inside wires andelectrodes or buttons may be neglected and that there is currentcontinuity on both sides of the electrode or button, that is, inside thetool and in the wellbore fluid.

Introduced current I and electric field U in chamber 8 are measured. Forlarge phase shifts, large Δφ, phase sensitive detection and processingtechniques are applied to determine formation characteristics, such asformation resistivity, R_(t). For small phase shifts, small Δφ, theimpedance amplitudes are used to characterize the formation. Thistechnique is applicable for both water and oil-base mud.

The foregoing description is presented for purposes of illustration anddescription, and is not intended to limit the invention to the formsdisclosed herein. Consequently, variations and modificationscommensurate with the above teachings and the teaching of the relevantart are deemed to reside within the spirit and scope of the invention asclaimed and described.

1. A device for measuring characteristics of a geologic formation usinga floating reference signal comprising: a mud chamber; an electrodedisposed within said mud chamber; an electrically conductive platedisposed within said mud chamber, said plate separated from saidelectrode, whereby an electric field may be maintained between saidconductive plate and said electrode; and an opening in said mud chamber,thereby allowing drilling fluids to pass there through.
 2. The device ofclaim 1 further comprising an alternating current source, saidalternating current source in electrical connection with said electrode.3. The device of claim 1 wherein said plate is an electricallyconductive lattice.
 4. The device of claim 1 wherein the distance ofseparation between said electrode and said plate is less than thesmallest lineal surface dimension of said electrode.
 5. A device formeasuring characteristics of a geologic formation in a wellbore using afloating reference signal comprising: a plurality of electrodes; aplurality of electrically conductive plates, each of said plurality ofelectrically conductive plates positioned respective to at least one ofsaid plurality of electrodes, wherein the plates of said plurality ofelectrically conductive plates are separated from each other by aninsulating material; and a mud chamber located between said plurality ofelectrodes and said plurality of electrically conductive plates; andwherein said plurality of electrodes and said plurality of plates areelectrically isolated from each other except through any contentscontained in said mud chamber; and said mud chamber is in fluidcommunication with said wellbore, thereby allowing wellbore fluids topass there through.
 6. A method for characterizing a geologic formationcomprising the steps of: determining the electrical properties of acurrent introduced into the geologic formation from a wellbore, therebyproducing a formation measurement; determining the electrical propertiesof said current introduced into the geologic formation through a portionof said wellbore, thereby producing a reference signal; anddifferentiating said formation measurement from said reference signal.7. The method of claim 6 wherein said differentiation step comprises thestep of: determining the phase difference between said formationmeasurement and said reference signal.
 8. The method of claim 7 whereinsaid step of determining the phase difference between said formationmeasurement and said reference signal comprises the step of: measuringthe voltage of said introduced current when the voltage of saidreference signal is approximately zero.
 9. The method of claim 6 whereinsaid differentiation step comprises the step of: determining theimpedance amplitude of said formation measurement.
 10. The method ofclaim 6 wherein said differentiation step comprises the steps of:determining the phase difference between said formation measurement andsaid reference signal, whereby a measurement of the phase difference isproduced; and selecting between measurement of the phase differencebetween said formation measurement and said reference signal for largephase differences and measurement of the impedance amplitude of saidformation measurement for small phase differences, whereby acharacterization of the geologic formation is produced.
 11. The methodof claim 6 wherein said introduced current is provided through at leastone of a plurality of electrodes disposed in the wellbore.
 12. Themethod of claim 6 wherein said introduced current is an alternatingcurrent.
 13. The method of claim 6 wherein: said step of determining theelectrical properties of a current introduced into the geologicformation from a wellbore comprises the steps of: introducing analternating electrical current into the wellbore using an electrode inelectrical contact with the wellbore fluids, thereby inducing a currentinto the geologic formation; measuring the alternating voltage of saidintroduced alternating electrical current, thereby producing a firstmeasured voltage; and measuring the current of said introducedalternating electrical current, thereby producing a first measuredcurrent; and said determining the electrical properties of said currentintroduced into the geologic formation through a portion of saidwellbore comprises the step of: measuring the differential alternatingvoltage of said introduced alternating electrical current between saidelectrode and an electrically conductive plate disposed between saidelectrode and the geologic formation, thereby producing a secondmeasured voltage.
 14. The method of claim 13 wherein said step ofdifferentiating said formation measurement from said reference signalcomprises the step of correlating said first measured voltage with saidsecond measured voltage, thereby producing a phase shift measurement.15. The method of claim 13 wherein said step of differentiating saidformation measurement from said reference signal comprises the step ofdividing said first measured current into said first measured voltagewhen the non direct current component of said second measured voltage isapproximately zero.
 16. The method of claim 13 wherein said step ofdifferentiating said formation measurement from said reference signalcomprises the step of selecting at least between: correlating said firstmeasured voltage with said second measured voltage, thereby producing aphase shift measurement; and dividing said first measured current intosaid first measured voltage when the non direct current component ofsaid second measured voltage is approximately zero.